Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in printing industries, non-image areas are generally hydrophilic and image areas are generally oleophillic. Consequently, oil based inks are repelled from the non-image areas after water has been applied to the substrate.
Image and non-image areas can be created by processes which include a step of exposing a layer of image material on the surface of the substrate to radiation. The exposure to radiation creates solubility differences in the image material corresponding to image and non-image areas. During development, the more soluble areas are removed, leaving a pattern on the substrate corresponding to the image.
Preparation of the substrate for receiving a layer of the image material must ensure that the image material bonds to the substrate. However, it must allow release of the soluble image material during development.
In addition, other problems need to be addressed when preparing a printing plate. For example, it is desirable for the image material to change color when it is exposed. This is generally achieved by including a pH sensitive dye in the material of the image layer, the dye often being arranged to change color by reaction with acid produced during a photochemical reaction of the light sensitive material of the image layer. However, it has been found that properties of the substrate itself can affect the ability of the dye to change color. In addition, it is desirable for the dye not to stain the substrate to any significant degree since, otherwise, on development, such staining will be visible. This would be aesthetically unacceptable and, furthermore, would be commercially unacceptable since plate processing personnel would be unable to determine whether a plate had been fully developed.
It is also desirable for it to be possible to remove unwanted image areas from a printing plate after exposure, for example to correct an error, in order to prevent such areas inking in and, therefore, being printed. The ability to remove selected areas will depend, to some extent, on the adhesive force between the substrate and the material of the image layer.
It is an object of the present invention to address the above-described problems.